1. Field of the Invention
The present invention relates to a developing apparatus for use in an electrophotographic image forming machine and to an image forming machine incorporating the developing apparatus. More particularly, the present invention relates to a trickle developing apparatus that gradually supplies fresh developer and gradually discharge deteriorated developer and to an image forming machine incorporating the developing apparatus.
2. Description of the Related Art
As developing systems employed for electrophotographic image forming machinees, the one-component developing system in which toner is used as the main component of the developer and the two-component developing system in which toner and carrier are used as the main components of the developer are known.
The two-component developing system that uses toner and carrier, in which the toner and carrier are charged by friction contact therebetween to predetermined polarities, has a characteristic that the stress on the toner is less than that in the one-component developing system that uses a one-component developer. Since the surface area of the carrier is larger than that of the toner, the carrier is less contaminated with the toner attached to the surface thereof. However, with the use for a long period, contamination (spent) attached to the surface of the carrier increases, whereby the capability of charging the toner is reduced gradually. As a result, problems of photographic fog and toner scattering occur. Although it is conceivable that the amount of the carrier stored in a two-component developing apparatus is increased to extend the life of the developing apparatus, this is undesirable because the developing apparatus becomes larger in size.
To solve the problems encountered in the two-component developer, Patent document 1 discloses the so-called trickle developing apparatus being characterized in that fresh developer is gradually replenished into the developing apparatus and developer deteriorated in charging capability is gradually discharged from the developing apparatus, whereby the increase of the deteriorated carrier is suppressed. The developing apparatus is configured to maintain the volume level of the developer inside the developing apparatus approximately constant by discharging an excessive amount of deteriorated developer using the change in the volume of the developer. In the trickle developing apparatus, the deteriorated carrier inside the developing apparatus is gradually replaced with fresh carrier, and the charging performance of the carrier inside the developing apparatus can be maintained approximately constant.
In the trickle developing apparatus, since developer is replenished while the developer inside the developing apparatus is discharged, the amount of the developer existing inside the developing apparatus changes, and the amount of the developer existing inside the developing apparatus is not constant at all times. Hence, the trickle developing apparatus has a problem of causing a toner concentration detection error owing to the difference in the amount of the developer inside the developing apparatus even though the toner concentration is the same.
As main methods for detecting the toner concentration in the two-component developing system in which toner and carrier are used, an optical detection method for detecting the content ratio of toner per unit area by detecting the reflection amount of the light irradiated to developer and a magnetic detection method for detecting the content ratio of toner per unit volume by detecting the permeability of magnetic carrier are available. The magnetic detection method is generally used in view of the cost of a sensor itself and the staining properties of the sensor.
The magnetic detection method has a problem of causing an error in the detection of the toner concentration since the permeability in the detection area changes not only owing to the change in the toner concentration but also owing to bulk density because of the principle of the detection thereof.
Hence, to prevent errors from occurring in the detection of the toner concentration, Patent document 2 has proposed a technology in which the change in the toner concentration of developer and the change in the density thereof are detected using sensors based on different detection principles, such as an optical sensor and a magnetic sensor, and the correction amount corresponding to the change in density is added to the toner concentration obtained using the optical sensor.    [Patent document 1] Japanese Patent Application Laid-Open Publication No. Sho 59-100471    [Patent document 2] Japanese Patent Application Laid-Open Publication No. Hei 05-341654
However, in the technology disclosed in Patent document 2, since multiple sensors based on different detection principles are disposed, there are problems in which it is difficult to make the developing apparatus compact, the control method therefor is complicated, and the cost is high. Furthermore, in the technology disclosed in Patent document 2, the toner concentration obtained using an optical sensor is corrected to an appropriate toner concentration using the correction amount corresponding to the change in density, but the amount of the developer inside the developing apparatus is not estimated or detected.
Moreover, in the trickle developing apparatus in which the amount of the developer existing inside the developing apparatus changes, even if the toner concentration is detected accurately, there is a problem in which if a constant amount of developer is replenished continuously, the toner concentration inside the developing apparatus becomes different from an appropriate reference toner concentration. In other words, in the case that the amount of the developer existing inside the developing apparatus is small, if a constant amount of developer is replenished continuously, the replenishment amount of toner becomes too large, and the toner concentration inside the developing apparatus continues to be higher than the reference toner concentration. Conversely, in the case that the amount of the developer existing inside the developing apparatus is large, if a constant amount of developer is replenished continuously, the replenishment amount of toner becomes too small, and the toner concentration inside the developing apparatus continues to be lower than the reference toner concentration. Hence, in both cases, the toner concentration inside the developing apparatus becomes different from the appropriate reference toner concentration.
Accordingly, the technical problem to be solved by the present invention is to provide a developing apparatus and an image forming machine capable of carrying out excellent image formation for a long period by replenishing an appropriate amount of developer depending on toner concentration and the amount of developer for a trickle developing apparatus that uses a two-component developer.